Maine, Maryland, Minnesota and Washington state are expected to vote in November on allowing same-sex marriage. Proponents hope he'll help change minds; opponents say he won't.

But he predicted that Obama's endorsement would change relatively few votes on the ballot measure. Instead, he said the president's announcement may prompt a backlash, particularly among anti-Obama voters who feel "intimidated" by proponents of gay marriage.

In the past, greater intensity among same-sex marriage opponents has given them outsized strength at the ballot box. Wolfson, of Freedom to Marry, acknowledged that gaining a majority among the electorate as a whole is "an uphill fight for any minority group," which is why minority rights are not usually won by referendum but by court or legislative action.

Opposition to gay unions from African Americans has also bolstered proponents of traditional marriage. In 2008, more than 9 in 10 black voters in California backed Obama, then overwhelmingly voted for Proposition 8, the successful ballot measure to overturn the state Supreme Court's decision allowing same-sex marriage.

Derek McCoy, of the Maryland Marriage Alliance, which is attempting to invalidate that state's gay marriage law, predicted Obama's endorsement would sway few African Americans there, where they make up about one-fourth of the electorate.

"I've had several people call me that were Obama supporters, that voted for him, that said, 'I can't believe that he went all the way out there and did this,' " said McCoy, associate pastor of the 2,500-member Hope Christian Church in majority-black Prince Georges County outside Washington.

Both sides have used high-profile endorsements to influence public opinion on the emotionally charged issue. Former President Clinton stepped into the North Carolina fray when he recorded an automated phone call urging voters to reject the state's gay marriage ban. The Rev. Billy Graham, in a full-page newspaper ad, asked them to support it.

But strategists for gay marriage initiatives say the most powerful statements are more often delivered to undecided voters by friends and co-workers.

"Voters conflicted on this issue are struggling on a very personal level," said Amy Simon, a Democratic pollster in Oakland, who is advising the gay rights forces in Maine and Washington. "Having regular people talk about it is more connecting, because people identify more with everyday people and how they think about it."